| java.util.Map
| Map | | public interface Map (Code) | | An object that maps keys to values. A map cannot contain duplicate keys;
each key can map to at most one value.
This interface takes the place of the Dictionary class, which
was a totally abstract class rather than an interface.
The Map interface provides three collection views, which
allow a map's contents to be viewed as a set of keys, collection of values,
or set of key-value mappings. The order of a map is defined as
the order in which the iterators on the map's collection views return their
elements. Some map implementations, like the TreeMap class, make
specific guarantees as to their order; others, like the HashMap
class, do not.
Note: great care must be exercised if mutable objects are used as map
keys. The behavior of a map is not specified if the value of an object is
changed in a manner that affects equals comparisons while the
object is a key in the map. A special case of this prohibition is that it
is not permissible for a map to contain itself as a key. While it is
permissible for a map to contain itself as a value, extreme caution is
advised: the equals and hashCode methods are no longer
well defined on such a map.
All general-purpose map implementation classes should provide two
"standard" constructors: a void (no arguments) constructor which creates an
empty map, and a constructor with a single argument of type Map,
which creates a new map with the same key-value mappings as its argument.
In effect, the latter constructor allows the user to copy any map,
producing an equivalent map of the desired class. There is no way to
enforce this recommendation (as interfaces cannot contain constructors) but
all of the general-purpose map implementations in the JDK comply.
The "destructive" methods contained in this interface, that is, the
methods that modify the map on which they operate, are specified to throw
UnsupportedOperationException if this map does not support the
operation. If this is the case, these methods may, but are not required
to, throw an UnsupportedOperationException if the invocation would
have no effect on the map. For example, invoking the
Map.putAll(Map) method on an unmodifiable map may, but is not required to, throw the
exception if the map whose mappings are to be "superimposed" is empty.
Some map implementations have restrictions on the keys and values they
may contain. For example, some implementations prohibit null keys and
values, and some have restrictions on the types of their keys. Attempting
to insert an ineligible key or value throws an unchecked exception,
typically NullPointerException or ClassCastException.
Attempting to query the presence of an ineligible key or value may throw an
exception, or it may simply return false; some implementations will exhibit
the former behavior and some will exhibit the latter. More generally,
attempting an operation on an ineligible key or value whose completion
would not result in the insertion of an ineligible element into the map may
throw an exception or it may succeed, at the option of the implementation.
Such exceptions are marked as "optional" in the specification for this
interface.
This interface is a member of the
Java Collections Framework.
Many methods in Collections Framework interfaces are defined
in terms of the
Object.equals(Object) equals method. For
example, the specification for the
Map.containsKey(Object)containsKey(Object key) method says: "returns true if and
only if this map contains a mapping for a key k such that
(key==null ? k==null : key.equals(k))." This specification should
not be construed to imply that invoking Map.containsKey
with a non-null argument key will cause key.equals(k) to
be invoked for any key k. Implementations are free to
implement optimizations whereby the equals invocation is avoided,
for example, by first comparing the hash codes of the two keys. (The
Object.hashCode specification guarantees that two objects with
unequal hash codes cannot be equal.) More generally, implementations of
the various Collections Framework interfaces are free to take advantage of
the specified behavior of underlying
Object methods wherever the
implementor deems it appropriate.
<
Parameters:
K - > the type of keys maintained by this map<
Parameters:
V - > the type of mapped values
author:
Josh Bloch
version:
1.62, 05/05/07
See Also: HashMap
See Also: TreeMap
See Also: Hashtable
See Also: SortedMap
See Also: Collection
See Also: Set
since:
1.2 |
| Inner Class :interface Entry | |
| Method Summary | |
| void | clear()
Removes all of the mappings from this map (optional operation). | | boolean | containsKey(Object key)
Returns true if this map contains a mapping for the specified
key. | | boolean | containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value. | | Set<Map.Entry<K, V>> | entrySet()
Returns a
Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. | | boolean | equals(Object o)
Compares the specified object with this map for equality. | | V | get(Object key)
Returns the value to which the specified key is mapped,
or
null if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k to a value
v such that
(key==null ? k==null : key.equals(k)) , then this method returns
v ; otherwise
it returns
null . | | int | hashCode()
Returns the hash code value for this map. | | boolean | isEmpty()
Returns true if this map contains no key-value mappings. | | Set<K> | keySet()
Returns a
Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. | | V | put(K key, V value)
Associates the specified value with the specified key in this map
(optional operation). | | void | putAll(Map<? extends K, ? extends V> m)
Copies all of the mappings from the specified map to this map
(optional operation). | | V | remove(Object key)
Removes the mapping for a key from this map if it is present
(optional operation). | | int | size()
Returns the number of key-value mappings in this map. | | Collection<V> | values()
Returns a
Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. |
| clear | | void clear()(Code) | | Removes all of the mappings from this map (optional operation).
The map will be empty after this call returns.
throws:
UnsupportedOperationException - if the clear operationis not supported by this map |
| containsKey | | boolean containsKey(Object key)(Code) | | Returns true if this map contains a mapping for the specified
key. More formally, returns true if and only if
this map contains a mapping for a key k such that
(key==null ? k==null : key.equals(k)). (There can be
at most one such mapping.)
Parameters:
key - key whose presence in this map is to be tested true if this map contains a mapping for the specifiedkey
throws:
ClassCastException - if the key is of an inappropriate type forthis map (optional)
throws:
NullPointerException - if the specified key is null and this mapdoes not permit null keys (optional) |
| containsValue | | boolean containsValue(Object value)(Code) | | Returns true if this map maps one or more keys to the
specified value. More formally, returns true if and only if
this map contains at least one mapping to a value v such that
(value==null ? v==null : value.equals(v)). This operation
will probably require time linear in the map size for most
implementations of the Map interface.
Parameters:
value - value whose presence in this map is to be tested true if this map maps one or more keys to thespecified value
throws:
ClassCastException - if the value is of an inappropriate type forthis map (optional)
throws:
NullPointerException - if the specified value is null and thismap does not permit null values (optional) |
| entrySet | | Set<Map.Entry<K, V>> entrySet()(Code) | | Returns a
Set view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation, or through the
setValue operation on a map entry returned by the
iterator) the results of the iteration are undefined. The set
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll and
clear operations. It does not support the
add or addAll operations.
a set view of the mappings contained in this map |
| equals | | boolean equals(Object o)(Code) | | Compares the specified object with this map for equality. Returns
true if the given object is also a map and the two maps
represent the same mappings. More formally, two maps m1 and
m2 represent the same mappings if
m1.entrySet().equals(m2.entrySet()). This ensures that the
equals method works properly across different implementations
of the Map interface.
Parameters:
o - object to be compared for equality with this map true if the specified object is equal to this map |
| get | | V get(Object key)(Code) | | Returns the value to which the specified key is mapped,
or
null if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k to a value
v such that
(key==null ? k==null : key.equals(k)) , then this method returns
v ; otherwise
it returns
null . (There can be at most one such mapping.)
If this map permits null values, then a return value of
null does not necessarily indicate that the map
contains no mapping for the key; it's also possible that the map
explicitly maps the key to
null . The
Map.containsKeycontainsKey operation may be used to distinguish these two cases.
Parameters:
key - the key whose associated value is to be returned the value to which the specified key is mapped, or null if this map contains no mapping for the key
throws:
ClassCastException - if the key is of an inappropriate type forthis map (optional)
throws:
NullPointerException - if the specified key is null and this mapdoes not permit null keys (optional) |
| hashCode | | int hashCode()(Code) | | Returns the hash code value for this map. The hash code of a map is
defined to be the sum of the hash codes of each entry in the map's
entrySet() view. This ensures that m1.equals(m2)
implies that m1.hashCode()==m2.hashCode() for any two maps
m1 and m2, as required by the general contract of
Object.hashCode .
the hash code value for this map
See Also: Map.Entry.hashCode
See Also: Object.equals(Object)
See Also: Map.equals(Object) |
| isEmpty | | boolean isEmpty()(Code) | | Returns true if this map contains no key-value mappings.
true if this map contains no key-value mappings |
| keySet | | Set<K> keySet()(Code) | | Returns a
Set view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. If the map is modified
while an iteration over the set is in progress (except through
the iterator's own remove operation), the results of
the iteration are undefined. The set supports element removal,
which removes the corresponding mapping from the map, via the
Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations. It does not support the add or addAll
operations.
a set view of the keys contained in this map |
| put | | V put(K key, V value)(Code) | | Associates the specified value with the specified key in this map
(optional operation). If the map previously contained a mapping for
the key, the old value is replaced by the specified value. (A map
m is said to contain a mapping for a key k if and only
if
Map.containsKey(Object) m.containsKey(k) would return
true.)
Parameters:
key - key with which the specified value is to be associated
Parameters:
value - value to be associated with the specified key the previous value associated with key, ornull if there was no mapping for key.(A null return can also indicate that the mappreviously associated null with key,if the implementation supports null values.)
throws:
UnsupportedOperationException - if the put operationis not supported by this map
throws:
ClassCastException - if the class of the specified key or valueprevents it from being stored in this map
throws:
NullPointerException - if the specified key or value is nulland this map does not permit null keys or values
throws:
IllegalArgumentException - if some property of the specified keyor value prevents it from being stored in this map |
| putAll | | void putAll(Map<? extends K, ? extends V> m)(Code) | | Copies all of the mappings from the specified map to this map
(optional operation). The effect of this call is equivalent to that
of calling
Map.put(Object,Object) put(k, v) on this map once
for each mapping from key k to value v in the
specified map. The behavior of this operation is undefined if the
specified map is modified while the operation is in progress.
Parameters:
m - mappings to be stored in this map
throws:
UnsupportedOperationException - if the putAll operationis not supported by this map
throws:
ClassCastException - if the class of a key or value in thespecified map prevents it from being stored in this map
throws:
NullPointerException - if the specified map is null, or ifthis map does not permit null keys or values, and thespecified map contains null keys or values
throws:
IllegalArgumentException - if some property of a key or value inthe specified map prevents it from being stored in this map |
| remove | | V remove(Object key)(Code) | | Removes the mapping for a key from this map if it is present
(optional operation). More formally, if this map contains a mapping
from key k to value v such that
(key==null ? k==null : key.equals(k)), that mapping
is removed. (The map can contain at most one such mapping.)
Returns the value to which this map previously associated the key,
or null if the map contained no mapping for the key.
If this map permits null values, then a return value of
null does not necessarily indicate that the map
contained no mapping for the key; it's also possible that the map
explicitly mapped the key to null.
The map will not contain a mapping for the specified key once the
call returns.
Parameters:
key - key whose mapping is to be removed from the map the previous value associated with key, ornull if there was no mapping for key.
throws:
UnsupportedOperationException - if the remove operationis not supported by this map
throws:
ClassCastException - if the key is of an inappropriate type forthis map (optional)
throws:
NullPointerException - if the specified key is null and thismap does not permit null keys (optional) |
| size | | int size()(Code) | | Returns the number of key-value mappings in this map. If the
map contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
the number of key-value mappings in this map |
| values | | Collection<V> values()(Code) | | Returns a
Collection view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. If the map is
modified while an iteration over the collection is in progress
(except through the iterator's own remove operation),
the results of the iteration are undefined. The collection
supports element removal, which removes the corresponding
mapping from the map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll and clear operations. It does not
support the add or addAll operations.
a collection view of the values contained in this map |
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